9356
J. Am. Chem. Soc. 1998, 120, 9356-9361
Accelerating the Kinetics of Thiol Self-Assembly on GoldsA Spatial Confinement Effect Song Xu,† Paul E. Laibinis,‡ and Gang-yu Liu*,† Contribution from the Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202, and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed June 3, 1998
Abstract: The adsorption of alkanethiols onto gold surfaces to form self-assembled monolayers (SAMs) occurs more than 10 times faster in a spatially confined environment than on unconfined bare substrates, and the adsorbed layers exhibit higher coverage and two-dimensional crystallinity. The spatially constrained reaction environment is prepared with use of an atomic force microscope tip to displace thiols within a previously formed SAM. During the displacement, the thiol molecules present in the solution above the SAM rapidly assemble onto the exposed nanometer-size gold area that is confined by the scanning tip and surrounding SAM. The accelerated rate is attributed to a change in the pathway for the self-assembly process as the spatial confinement makes it geometrically more probable and energetically more favorable for the initially adsorbed thiols to adopt a standing-up configuration directly in this microenvironment. In contrast, thiols that self-assemble onto gold surfaces in an unconstrained environment initially form a lying-down phase, which subsequently degrades and forms a standing-up phase. Our observations suggest that spatial confinement can provide an effective means to change the mechanism and kinetics of certain surface reactions by sterically preventing alternative reaction pathways and stabilizing particular transition states or reaction intermediates. In addition, the results underlie the development of a new method (“nanografting”) for patterning SAMs laterally with nanometer-level precision.
Introduction Self-assembled monolayers (SAMs) offer many promising applications in the developments of boundary lubricants, anticorrosion coatings, and recently the microfabrication process because they can be laterally patterned and then used as resists for pattern transfer.1-3 To be effective as a resist, the SAM should be stable and contain few defects. A popular system for these applications has been SAMs derived from the adsorption of alkanethiols onto metals such as gold, silver, and copper.4 These thiol-derived SAMs contain ordered domains that are separated by boundaries (areas of lower surface coverage) and various defects,1,5,6 whose size and distribution depend on the interplay of kinetic and thermodynamic factors * To whom correspondence should be addressed. † Wayne State University. ‡ Massachusetts Institute of Technology. (1) Ulman, A. An Introduction to Ultrathin Organic FilmssFrom Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (2) (a) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (b) Huang, J. Y.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (c) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (3) (a) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (b) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274. (c) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024. (4) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 448. (b) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (5) (a) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (b) Poirier, G. E.; Tarlov, M. J.; Rushmeierf, H. E.; Langmuir 1994, 10, 3383. (c) Sondag-Huthorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. Phys. Chem. 1994, 98, 6826. (d) McDermott, C. A.; McDermott, M. T.; Green, J. B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (e) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979.
during the growth of the SAM. Empirically, a thiol-derived SAM with a large domain size and low defect density can be prepared by contacting a gold surface with a dilute solution of the thiol for at least 24 h.1,4,7 A molecular level understanding of the self-assembly process and kinetics is of fundamental importance for improving the quality and usefulness of SAMs. As shown in recent studies using scanning tunneling microscopy (STM)8 and helium and X-ray diffraction,9 the selfassembly of these molecules from the vapor phase follows two major steps. First, the thiol molecules adsorb on gold and form a lattice-gas or mobile phase that gradually evolves into crystalline islands with the molecules oriented parallel to the gold surface.8,9 At the saturation coverage of this lying-down phase, a solid-to-solid-phase transition occurs to produce islands of molecules in a standing-up configuration.8 Using low-energy helium diffraction, Schwartz et al. revealed that the low-density lying-down monolayer degrades into a disordered state, from which the standing-up phase is formed.9 Using an ultrahigh vacuum STM, Poirier and Pylant provided a molecular-level in situ picture at each step of the self-assembly process for thiols onto gold from the vapor phase.8 In practice, most SAMs are prepared from a solution phase.1,4 Under these conditions, (6) (a) Camillone, N. C.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (b) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (c) Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P. J. Chem. Phys. 1997, 106, 1600. (7) (a) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (b) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (8) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (9) (a) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737. (b) Schwartz, P.; Schreiber, F.; Eisenberger, P.; Scoles, G. Phys. ReV. B 1998, preprint.
S0002-7863(98)01938-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/20/1998
Accelerating the Kinetics of Thiol Self-Assembly on Gold atomic force microscopy (AFM) allows surface reactions be followed in situ and in real time with high spatial resolution.10 In a previous study, we examined the mechanism and kinetics of self-assembly in solution using time-dependent AFM images acquired during the formation of SAMs.10 This work revealed a stepwise process for the formation of the SAM and the presence of a structural phase transition during its assembly.10 In general, chemical reactions can be accelerated by increases in the concentration of reactants and in the reaction temperature, or by using catalysts which often change the reaction mechanism. In certain gas-phase and solution-phase reactions, rate accelerations have been possible within microscopic environments such as the cavities of zeolites, and the capsules of proteins and various supramolecular complexes.11 The rate enhancements for these systems often result from the existence of encapsulating microenvironments that enthalpically stabilize specific reaction intermediates and may sterically favor the formation of a particular product.11 For the reaction processes that underlie the self-assembly of thiols onto gold to form densely packed monolayers, these processes can be accelerated by increases in the concentration of thiols or in the reaction temperature, or by changes in the polarity of the solvent medium.1,7,10 In general, the layers formed at these accelerated rates typically have small domain sizes and high defect densities. In the present study, we reveal that the self-assembly process is accelerated within a spatially confined environment and proceeds without sacrificing the quality of the resulting monolayer. The spatially confined environment is produced during a fabrication process called nanografting12 in which a gold surface is spatially confined by surrounding adsorbed thiols and an AFM tip. We have systematically investigated and compared the kinetics of self-assembly of thiols from solution onto freshly prepared gold surfaces and under spatially confined conditions. When the exposed gold area is sufficiently small, the process for SAM formation is at least 10 times faster than the corresponding unconfined process. In addition, SAMs formed under such spatial constraint have a higher coverage and crystallinity than the corresponding layers formed on an unconstrained gold surface. These observations appear to reflect a change in the self-assembly pathway that directly results from the spatial confinement. Experimental Method The AFM employs a home-constructed, deflection-type scanning head that exhibits high mechanical stability and a liquid cell that allows injection of solutions with minimal disturbance during in situ imaging.10 The scanner was controlled by an AFM100 preamplifier and STM1000 electronics manufactured by RHK Technology. Sharpened Si3N4 microlevers (Park Scientific Instruments) with a force constant of 0.1 N/m were used for AFM imaging. Gold (Alfa Aesar, 99.999%) was deposited in a high-vacuum evaporator (Denton Vacuum Inc., Model DV502-A) at a base pressure of ca. 10-7 Torr onto freshly cleaved mica substrates (clear ruby muscovite, Mica New York Corp.). The mica was preheated to 325 °C before deposition by using two quartz lamps mounted behind the mica to enhance the formation of terraced Au(111) domains.13 Typical (10) Xu, S.; Cruchon-Dupeyrat, S.; Garno, J.; Liu, G. Y.; Jennings, G. K.; Yong T.-H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002. (11) (a) Tokuna, Y.; Rebek, J. J. Am. Chem. Soc. 1998, 120, 66. (b) Kang, J. M.; Rebek, J. Nature 1996, 382, 239. (12) Xu, S.; Liu, G. Y. Langmuir 1997, 13, 127. (13) (a) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 39. (b) Wagner, P.; Hegner, M.; Guntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867. (c) Chidsey, C. E. D.; Loaicano, D. N.; Sleator, T.; Nakahaza, S. Surf. Sci. 1988, 200, 45. (d) Lang, C. A.; Dovek, M. M.; Nogami, J.; Quate, C. F. Surf. Sci. Lett. 1989, 224, 1974. (e) Wo¨ll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. ReV. B 1989, 39, 7988.
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9357 evaporation rates were 3 Å/s, and the thickness of the gold films ranged from 1500 to 2000 Å. n-Alkanethiols CH3(CH2)nSH (abbreviated to Cn+1SH) were purchased from Aldrich and used as received, except for 1-docosanethiol (C22SH) and 19-(octadecyloxy)nonadecanethiol (C18OC19SH) that were available from a previous study.10
Results and Discussion Self-Assembly of Alkanethiols from Solution onto Gold Surfaces. Figure 1 displays nine selected images representing critical moments during the self-assembly of C18SH from a 0.2 mM solution onto a freshly prepared gold thin film. Initially, the adsorbed thiol molecules align parallel to the surface (see Figure 1, parts B, B′, C, and C′). As adsorption proceeds, elevated islands ranging from 20 to 100 Å in lateral dimension appear in image 1D, where the height difference between these islands and the lying-down phase is 15 ( 2 Å (Figure 1D′). The height measurements in these topographic images suggest that these islands contain thiols in a standing-up configuration. With continued exposure to the thiols, the coverage of the standing-up phase increases through both nucleation and the growth of nuclei (Figure 1D-I). From this series of timedependent in situ AFM images, we conclude that the selfassembly of thiols onto gold from solution follows a similar mechanism to that from the vapor phase as revealed from investigations using STM8 and helium and X-ray diffraction.9 In the 0.2 mM thiol solution, a nearly complete monolayer (∼95%) formed in ∼30 min and contained domains of closepacked thiol molecules5,6 separated by “dark scars” that slowly heal over time periods of 24-96 h.10 These scars are likely to be gold areas that are covered by weakly adsorbed species that became trapped during the growth of the SAM. Self-Assembly of Alkanethiols onto Gold under Spatial Confinement. To study self-assembly under spatially constrained conditions, we follow a three-step process called nanografting.12 First, AFM is used to image a previously formed monolayer (matrix SAM) in a solution containing a desired thiol, and then the tip is positioned at a selected site. Second, the load is slowly increased to slightly above the displacement threshold for thiol adsorbates.14 During the scan, the AFM tip displaces the matrix thiols underneath the tip and exposes the Au(111) surface to the thiol solution;15 we refer to this step as nanoshaving. This displacement creates a transient microenvironment in which the freshly exposed gold is spatially constrained by the surrounding thiols and the AFM tip. Selfassembly of thiols from solution onto the newly exposed gold areas occurs within this reaction environment to form a SAM. Finally, the growth of the new monolayer is monitored at a reduced imaging force.12 For clarity, we refer to self-assembly that occurs during nanoshaving as spatially confined (or constrained) self-assembly (SCSA) and adsorption that occurs by immersing freshly prepared gold substrates into thiol solutions (as in Figure 1) as unconstrained self-assembly. Nanografting experiments with a C18S matrix in a 0.2 mM C18SH solution (Figure 2) produced two important observations. First, SCSA occurs faster than unconstrained growth in common thiol solutions. From the images taken immediately before and after the nanoshaving step (Figure 2, parts A and B), SAMs formed in less than 2.5 min. Our experimental approach in this case was not sufficiently fast to follow the SCSA. Most likely, (14) (a) Liu, G. Y.; Salmeron, M. B. Langmuir 1994, 10, 367. (b) Liu, G. Y.; Fenter, P.; Eisenberger, P.; Chidsey, C. E. D.; Ogletree, D. F.; Salmeron, M. B. J. Chem. Phys. 1994, 101, 4301. (15) The displaced thiols may dissolve into the solvent as RSSR, RSAun, RSH, or RSO3-.
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Figure 1. (A-I) In situ AFM images of the self-assembly of C18SH onto a freshly prepared Au(111) thin film from a 2-butanol solution. (B′-D′) Cross-sectional profiles for the corresponding line traces in images B-D. The concentration of C18SH was 0.2 mM. An elevated island (the bright spot) and several Au(111) single atomic steps in the lower left region of the image provided landmarks for in situ imaging.
Figure 2. (A) A 150 × 150 nm2 topographic scan of a C18S SAM at an imaging force of 0.1 nN. The Au(111) terraces are separated by five single-atom steps. (B) After nanografting in the central 50 × 50 nm2 area with a displacement force of 1.2 nN and a speed of 250 nm/s in a 0.2 mM C18SH solution. Note that the scars in part A are absent in the central area in part B. The boundary between the matrix and grafted areas can be identified easily because of the structural discontinuity. Etch pits are more clearly resolved in (B) because the AFM tip is sharpened during the high force scan or shaving process. (C) High-resolution image acquired by zooming into the region indicated in (B).
adsorption occurred following the shaving track of the AFM tip. The second observation is the scar-free morphology and long-range order of the newly formed SAM. Image 2B reveals that this newly formed SAM exhibits a similar thickness and
surface morphology as the surrounding matrix that was formed by soaking a gold thin film in a 1 mM thiol solution for at least 72 h. As a comparison, the monolayer formed on bare gold after soaking for 30 min in a 0.2 mM thiol solution (Figure 1I)
Accelerating the Kinetics of Thiol Self-Assembly on Gold Table 1.
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9359
Reaction Times (t) to Reach a Saturation Coverage (θ) during the Formation of Monolayers unconstrained reaction
spatially constrained t (min) for θ ≈ 95%
0.2 mM C18SH + Au(111)
30 ( 1
0.1 mM C22SH + Au(111)
30 ( 1
2 µM C18OC19SH + Au(111)
50 ( 2
reaction 0.2 mM C18SH + Au(111) in C18S/Au(111) C10S/Au(111) C10S/Au(111) HOC2S/Au(111) 0.1 mM C22SH + Au(111) in C22S/Au(111) C18S/Au(111) C18S/Au(111) C10S/Au(111) 2 µM C22SH + Au(111) in C18S/Au(111) 2 µM C18OC19SH + Au(111) in C18OC19S/Au(111)
t (min)a for θ ≈ 100%
area (nm2)
